If you've only had penetrative anal intercourse 3-4 times before, I would stay away from the sling unless you're in a more private setting (such as a private room in the establishment where you are) with a guy you trust.

Slings aren't amateur. Getting into them can require a certain level of skill and the whole point of them is really to make it so that the thrusting is faster and deeper, since the guy being penetrated isn't static and can get swung around on someone's dick. If you aren't used to anal sex, this could be problematic.

Further, if you're in a more public setting, your position in the sling may be perceived as an invitation for guys to line up for their turn, and again, as an amateur, that would be unwise.